Different people have different levels of willpower and fearlessness, and different and thresholds of motivation and temptation. We had a question within the last few months from a woman who was desperately afraid she was going to lose control and sleep with the neighbor, ruining both of their marriages, if she didn't find a way to put some space between them. On the reverse, we have people who can barely find the courage to look at someone of the opposite sex for more than a second for fear of hypothetical repercussions. Some people can't go a week without sex, while for others, "The Forty-Year-Old Virgin" is not far off of their life's story.
As for this guy, he apparently has a very low threshold for distraction, and seems not to have (or at least use) all that much willpower to fight it. If she's going to stay with him, she'll either have to live with it, or get him to get this under control. Given how much it bothers her, the "live with it" option seems rather unlikely.
Asking him, "Is there anything I can do to help you stay focused?" might help get things on the right track.
Both reward and punishment approaches have at least received lip service here. A combination of both might be in order. If he drops what he's doing to stare, an immediate *positive* reminder of where his attention should be might be possible under the table. Sitting across, a gentle touch of foot to leg might be enough to draw him back. Sitting side-by-side, hand on arm. If that doesn't work, or if it only encourages him, *that's* when you go with toe or heel to shin, or elbow to ribs, or fingernails in thigh.